1. Field of Invention
The present invention relates to an emission control method and apparatus of an internal combustion engine that controls emissions via a NOx storage device provided in an exhaust passage of the engine.
2. Description of Related Art
Direct injection type internal combustion engines are conventionally known in which the combustion mode is changed between a lean bum mode in which the engine air-fuel ratio is set to a fuel-lean side of the stoichiometric air-fuel ratio and a stoichiometric burn mode in which the engine air-fuel ratio is set to the stoichiometric air-fuel ratio. For example, according to an art disclosed in Japanese Patent Application Laid-Open No. 7-139340, NOx produced during a lean burn operation of an internal combustion engine is absorbed and stored into a NOx storage-reduction catalyst and, when the amount of NOx stored in the NOx storage-reduction catalyst reaches or exceeds an allowable value, a rich spike control of temporarily setting the engine air-fuel ratio to a fuel-rich ratio is performed. Due to the rich spike control, NOx stored in the NOx storage-reduction catalyst is reduced, so that NOx emission deterioration is prevented.
However, the aforementioned art becomes unable to efficiently reduce NOx by the rich spike control when the amount of exhaust gas becomes great and, therefore, the amount of exhaust gas flowing through the NOx storage-reduction catalyst also becomes great, for example, during high-speed engine operation or the like. If in this situation, a sufficient recovery of the NOx storing capability of the NOx storage-reduction catalyst is attempted, the rich spike control is performed for a long time, resulting in deteriorated HC and CO emissions.
To avoid this problem, it is conceivable to limit the execution time of the rich spike control (rich spike duration Tr) as indicated in FIG. 4. However, limitation of the rich spike duration Tr results in insufficient recovery of the NOx storing capability of the NOx storage-reduction catalyst, that is, a state in which the amount of NOx stored in the NOx storage-reduction catalyst is not xe2x80x9c0xe2x80x9d, when the lean burn operation is resumed. As a result, the amount of NOx that passes through the NOx storage-reduction catalyst during the lean burn operation increases, thus giving rise to a problem of NOx emission deterioration.
It is an object of the invention to provide an internal combustion engine emission control method and apparatus capable of curbing deterioration of NOx emission while avoiding deterioration of HC and CO emissions even when the amount of exhaust gas becomes great, for example, during a high-speed engine operation or the like.
An emission control method and apparatus of an internal combustion engine in accordance with a first aspect of the invention includes a storage device (or NOx storage-reduction catalyst) provided in an exhaust passage of the internal combustion engine that stores a threshold level of NOx, and reduces NOx stored in the storage device to recover a NOx storing level of the storage device by performing a rich spike control of temporarily shifting an engine air-fuel ratio to a fuel-rich ratio during a lean burn operation of the internal combustion engine. The emission control method and apparatus limits an execution time of the rich spike control, and performs a stoichiometric burn operation in which the engine air-fuel ratio is stoichiometric, after the limited execution time elapses.
In accordance with the aforementioned aspect, limitation of the execution time of the rich spike control prevents deterioration of emissions of hydrocarbons (HC) and CO (carbon monoxide) caused by prolonged rich spike control. Furthermore, since the stoichiometric burn operation is performed after the elapse of the limited execution time of the rich spike control, it is possible to recover the NOx storing level of the storage device while avoiding deterioration of HC and CO emissions owing to the emission control effect of the storage device during the stoichiometric burn operation. The recovery is, in other words, reduction of NOx remaining in the storage device at the end of the rich spike control accomplished by reducers contained in exhaust gas during the stoichiometric burn operation, such as HC, CO, etc. Therefore, it becomes possible to curb deterioration of NOx emission while avoiding deterioration of HC and CO emissions even when the amount of exhaust gas becomes great, for example, during a high-speed engine operation or the like.
In the aforementioned aspect, the exhaust passage may be provided with a three-way catalyst.
Therefore, since the exhaust passage is provided with the three-way catalyst and the storage device, the emission control effect of the three-way catalyst is added to the emission control effect of the storage device during the stoichiometric burn operation. Due to the emission control effects of the two catalysts, deterioration of HC and CO emissions can be more efficiently prevented. Hence, the NOx storing level can be recovered while deterioration of HC and CO emissions is more effectively avoided.
In the aforementioned aspect, the control may switch to the stoichiometric burn operation when the limited execution time of the rich spike control elapses without the NOx storing level having been recovered.
Therefore, the amount of NOx remaining in the storage device at the end of the rich spike control can be reduced during the stoichiometric burn operation while deterioration of the HC and CO emissions is avoided, owing to the emission control effect of the storage device or the emission control effects of the storage device and the three-way catalyst during the stoichiometric burn operation. Hence, the NOx storing level of the storage device can be sufficiently recovered before the lean burn operation is resumed.
In the aforementioned aspect, the control may switch from the stoichiometric burn operation to the lean burn operation after the NOx storing level is recovered.
Therefore, when the operation is switched from the stoichiometric bum operation to the lean burn operation, the NOx storing level has been recovered. Therefore, deterioration of NOx emission can be further curbed. Furthermore, the time before the next execution of the rich spike control, that is, the interval of execution of the rich spike control, can be increased in comparison with the conventional art, and deterioration of HC and CO emissions can be correspondingly curbed.
In the aforementioned aspect, the control may perform the stoichiometric burn operation for a pre-set period of time, and may switch to the lean burn operation after performing the stoichiometric burn operation.
Therefore, by performing the stoichiometric burn operation for a pre-set period of time, for example, a time that is sufficient for the stoichiometric burn operation to recover the NOx storing level, the NOx storing level can be recovered before the operation is switched to the lean burn operation. Hence, deterioration of NOx emission can be further curbed, and the interval of execution of the rich spike control can be increased from the conventional interval, so that, correspondingly, deterioration of HC and CO emissions can be further curbed.